As used herein, the term programmable logic device (PLD) refers to any integrated circuit that can be configured to perform a customized function. This term encompasses programmable read-only memories (PROMs), programmable logic arrays (PLAs), programmable array logic (PAL), and field-programmable gate arrays (FPGAs). Of particular interest to the present disclosure are the FPGAs.
FPGAs are integrated circuits that include a configurable array of logic gates. A programmer can configure an FPGA to implement nearly any desired function of moderate complexity. Some examples of suitable functions include counters, multipliers, filters, and state machines. System designers often find FPGAs to be useful because the FPGAs can be programmed in the field to perform functions that are custom-tailored to the designers' needs. Many FPGAs are volatile, that is, they lose their configuration when power is removed. Volatile FPGAs consequently need to be reconfigured when the power is restored. The configuration of FPGAs typically occurs via dedicated configuration pins.
When designing electronics for hazardous environments, one of the primary design goals for system designers is to minimize energy that is provided to the electronic circuitry so as to avoid any possibility of a spark or a high-temperature surface that could ignite flammable vapors. Underwriters Laboratories has provided a safety standard for electronic circuits being used in hazardous locations. This is the UL 913 Standard for Intrinsically Safe Apparatus and Associated Apparatus, which is hereby incorporated by reference. Among the concepts described therein are energy barriers. An energy barrier is a circuit located outside of the hazardous area that limits the voltage and current provided to the intrinsically safe circuitry located inside the hazardous area. Energy barriers are typically voltage and current limiting, fuse-protected shunt-diode circuits; optical isolators; and/or galvanic isolators.
Sensors (e.g. flow meters) are often needed in hazardous areas. Because it is often impractical to make monitoring and measurement electronics intrinsically safe as a whole, the bulk of the monitoring and measurement electronics are typically located outside the hazardous area or within an explosion-proof container. Only the sensing portion of the electronics is located in the hazardous area, and is designed to be intrinsically safe. The sensing portion of the electronics preferably communicates with the bulk of the electronics via a cable and an energy barrier.
The sensing portion may be a desirable place to use a FPGA because this would allow a significant degree of flexibility. For example, an FPGA could offer the ability to implement multiple functions with a single chip, the ability to upgrade software in the field, and the ability to customize the software logic to different field conditions. However, using an FPGA in the sensing portion creates a challenge, namely, a method of configuring the FPGA is needed.
One configuration method would be to connect a memory device such as an EEPROM to the programming pins of the FPGA. This method undesirably increases the cost of the sensing portion electronics. A method is needed that does not significantly increase the cost of the system.